The goal of the proposed research is to advance our understanding of higher brain functions by developing a new class of instrumentation that will enable investigators to simultaneously monitor neuronal activity from tens to hundreds of independently movable microelectrodes that are semi-chronically implanted in widespread regions of the brains of awake, behaving monkeys. To accomplish this objective, we plan to design and build an integrated system, consisting of a form-fitting chamber and a modular, replaceable guide array and microdrive, for implanting and independently manipulating up to 240 microelectrodes in the brains of alert macaque monkeys. The system will provide high density electrophysiological access to an entire cerebral hemisphere and enable simultaneous recording of unit and local field potential activity from many cortical and/or subcortical locations for periods lasting many weeks or months. We will implant and test the system in three macaque monkeys in order to i) optimize the design of the system and minimize the time and expense required for its manufacture, ii) determine the most effective design of the system to facilitate its surgical implantation, insure its mechanical stability, reduce the chances of intracranial infection, and provide for the best possible care of the wound margins surrounding the implant, iii) evaluate the long-term quality and stability of the electrophysiological recordings, and iv) evaluate the effects of the implanted electrodes and guide tubes on the histological structure of the brain tissue. This approach will insure that the system is both generic and modular in its design while optimizing its capabilities and minimizing the costs for its manufacture. When completed, the proposed system will provide the research community with a new technology for long-term monitoring of neuronal activity from distributed locations in the brain of non-human primates. This technological advance will open up vast new opportunities for studying the neural basis of higher brain functions.